Crossing the streams: When you're urinating and defecating at the same time, and you maneuver your body just right so you can urinate on your feces while it's coming out.
This isn't a sexual act, but I'm sure someone can figure out a way.
"Dude, I just pissed on my shit while it was still coming out. It was awesome!"
"Yeah man, I cross the streams all the time."